1. Field of Invention
The present invention relates to a boost DC/DC converter. More particularly, the present invention relates to a boost DC/DC converter applying the pulse modulation technology.
2. Description of Related Art
In power supply converting system, the load variation would correspondingly affect the power supply converting efficiency, so in case of full load or a big load, it is still preferred that the system maintains high efficiency and stable power conversion. As to application, the processor, the random access memory, the display or the hand phone etc., are certainly not in full load all the time, especially the mobile communication products, such as the hand phone, which is in power saving mode most of the time, therefore a power supply converting system which is able to maintain high efficiency under any load state is very important.
FIG. 1 is a schematic block diagram of a conventional boost DC/DC converter 100. When only the pulse wide modulation mode is used, the converter 100 comprises a switched boost circuit 110, a pulse width modulation circuit 120 and a load 130. Wherein, the switched boost circuit 110 comprises an inductor 111, a diode 112, a capacitor 113 and a power transistor 114. The power transistor 114 is switched on or off through pulse width modulation. When the power transistor 114 is on, the diode 112 is in reverse bias, and the electrical energy from an input voltage Vin1 is stored in the inductor 111, wherein the load's electrical energy is provided by the capacitor 113. When the power transistor 114 is off, the diode 112 is in forward bias, wherein the capacitor 113 and the load 130 absorb the electrical energy provided by the input voltage Vin1 and the inductor 111, thus Vout1>Vin1.
The pulse width modulation circuit 120 comprises a control feedback circuit, which includes an error amplifier 121, a triangular waveform generator 122, a pulse width modulation comparator 123 and a driver 124. After divided by the resistor R1 and R2 (Vout1×R2/(R1+R2)), the output voltage Vout1 of the switched boost circuit 110 compares with a reference voltage Vref1 by the error amplifier 121, and the pulse width modulation comparator 123 receives the output signal from the error amplifier 121 and compares with the output signal from the triangular waveform generator 122, a pulse width modulation signal PWM_CK is generated. And then the pulse width modulation signal PWM_CK is amplified through the driver 124 and then drives the power transistor 114.
Referring to FIG. 2, which is a relation diagram between the load current and the system efficiency of a boost DC/DC converter 100 shown in FIG. 1. When only the pulse wide modulation mode is used in the boost DC/DC converter 100, it's known from FIG. 2 that, in the boost DC/DC converter 100, when the load current IL1 is low (light load), the system efficiency is lower than when the load current IL1 is high (heavy load). It is because the pulse width modulation is controlled by fixed frequency. Even in light load, the power transistor 114 performs the switching at same frequency as in heavy load, thus consuming unnecessary switching power on the power transistor 114, increasing overall input power loss and reducing system efficiency. Therefore, a boost DC/DC converter capable of enhancing the system efficiency is desired.